Bring back the EliminateEmptyStatements transform for the minifier.

Originally this was part of module rehydration, and was removed
when module rehydration was deleted. However, I found that the
minifier had a similar need--after converting the program to IR, we
had occasional runs of `;;;;;;` where dead variables had been
eliminated and empty statements left behind in their place.

This transform is now used by sksl-minify, so it doesn't cost us any
code size or performance in Skia directly, but we can still get the
benefit in our minified modules.

Change-Id: Ibc38674df289b9baf1aa50f917d894fec5a20792
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/585967
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
6 files changed